ehci-atmel: Power down during suspend is normal

When an Atmel SoC is suspended with the backup mode, the USB bus will be
powered down. As this is expected, do not return an error to the driver
core when ehci_resume detects it.

Signed-off-by: Romain Izard <romain.izard.pro@gmail.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>